noun
: any of various devices to stop an undesirable motion or action: as
a. : an attachment for an elevator to prevent accidental falling
b. : a device that closes a supply valve and stops an engine in case of accident to the governor belt
c. : a contrivance to prevent a pulley tackle from overhauling
d. : a device in a lathe to prevent a carriage from colliding with the headstock
e. : a system for stopping trains automatically if they try to pass a stop signal
f. : a stop motion for textile machinery